1. Field
The present invention relates to metal-oxide-semiconductor (MOS) field effect transistors (FETs), and particularly to MOSFETs fabricated on Semiconductor-On-Insulator (“SOI”) and Silicon-On-Sapphire (“SOS”) substrates. In one embodiment, an SOI (or SOS) MOSFET is adapted to control accumulated charge and thereby improve linearity of circuit elements. In another embodiment, according to the present CIP, an SOI (or SOS) MOSFET is adapted to control accumulated charge and thereby improve gate oxide reliability.
2. Description of Related Art
Although the disclosed method and apparatus for use in improving the linearity of MOSFETs are described herein as applicable for use in SOI MOSFETs, it will be appreciated by those skilled in the electronic device design arts that the present teachings are equally applicable for use in SOS MOSFETs. The present teachings also apply to other semiconductor-on-insulator systems, wherein the silicon is replaced by another semiconductor such as silicon-germanium (SiGe). It will also be appreciated by those skilled in the electronic design arts that the present disclosed method and apparatus also apply to virtually any insulating gate technology, and to integrated circuits having a floating body. As those skilled in the art will appreciate, technologies are constantly being developed for achieving “floating body” implementations. For example, the inventors are aware of circuits implemented in bulk silicon wherein circuit implementations are used to “float” the body of the device. The disclosure contemplates embodiments of the disclosed method and apparatus implemented in any of the developing floating body implementations. Therefore, references to and exemplary descriptions of SOI MOSFETs herein are not to be construed as limiting the applicability of the present teachings to SOI MOSFETs only. Rather, as described below in more detail, the disclosed method and apparatus find utility in MOSFETs implemented in a plurality of device technologies, including SOS.
As is well known, a MOSFET employs a gate-modulated conductive channel of n-type or p-type conductivity, and is accordingly referred to as an “NMOSFET” or “PMOSFET”, respectively. FIG. 1 shows a cross-sectional view of an exemplary prior art SOI NMOSFET 100. As shown in FIG. 1, the prior art SOI NMOSFET 100 includes an insulating substrate 118 that may comprise a buried oxide layer, sapphire, or other insulating material. A source 112 and drain 116 of the NMOSFET 100 comprise N+ regions (i.e., regions that are heavily doped with an “n-type” dopant material) produced by ion implantation into a silicon layer positioned above the insulating substrate 118. (The source and drain of PMOSFETs comprise P+ regions (i.e., regions heavily doped with “p-type” dopant material)). The body 114 comprises a P− region (i.e., a region that is lightly doped with a “p-type” dopant), produced by ion implantation, or by dopants already present in the silicon layer when it is formed on the insulating substrate 118. As shown in FIG. 1, the NMOSFET 100 also includes a gate oxide 110 positioned over the body 114. The gate oxide 110 typically comprises a thin layer of an insulating dielectric material such as SiO2. The gate oxide 110 electrically insulates the body 114 from a gate 108 positioned over the gate oxide 110. The gate 108 comprises a layer of metal or, more typically, polysilicon.
A source terminal 102 is operatively coupled to the source 112 so that a source bias voltage “Vs” may be applied to the source 112. A drain terminal 106 is operatively coupled to the drain 116 so that a drain bias voltage “Vd” may be applied to the drain 116. A gate terminal 104 is operatively coupled to the gate 108 so that a gate bias voltage “Vg” may be applied to the gate 108.
As is well known, when a voltage is applied between the gate and source terminals of a MOSFET, a generated electric field penetrates through the gate oxide to the transistor body. For an enhancement mode device, a positive gate bias creates a channel in the channel region of the MOSFET body through which current passes between the source and drain. For a depletion mode device, a channel is present for a zero gate bias. Varying the voltage applied to the gate modulates the conductivity of the channel and thereby controls the current flow between the source and drain.
For an enhancement mode MOSFET, for example, the gate bias creates a so-called “inversion channel” in a channel region of the body 114 under the gate oxide 110. The inversion channel comprises carriers having the same polarity (e.g., “P” polarity (i.e., hole carriers), or “N” polarity (i.e., electron carriers) carriers) as the polarity of the source and drain carriers, and it thereby provides a conduit (i.e., channel) through which current passes between the source and the drain. For example, as shown in the SOI NMOSFET 100 of FIG. 1, when a sufficiently positive voltage is applied between the gate 108 and the source 112 (i.e. a positive gate bias exceeding a threshold voltage Vth), an inversion channel is formed in the channel region of the body 114. As noted above, the polarity of carriers in the inversion channel is identical to the polarity of carriers in the source and drain. In this example, because the source and drain comprise “n-type” dopant material and therefore have N polarity carriers, the carriers in the channel comprise N polarity carriers. Similarly, because the source and drain comprise “p-type” dopant material in PMOSFETs, the carriers in the channel of turned on (i.e., conducting) PMOSFETs comprise P polarity carriers.
Depletion mode MOSFETs operate similarly to enhancement mode MOSFETs, however, depletion mode MOSFETs are doped so that a conducting channel exists even without a voltage being applied to the gate. When a voltage of appropriate polarity is applied to the gate the channel is depleted. This, in turn, reduces the current flow through the depletion mode device. In essence, the depletion mode device is analogous to a “normally closed” switch, while the enhancement mode device is analogous to a “normally open” switch. Both enhancement and depletion mode MOSFETs have a gate voltage threshold, Vth, at which the MOSFET changes from an off-state (non-conducting) to an on-state (conducting).
No matter what mode of operation an SOI MOSFET employs (i.e., whether enhancement or depletion mode), when the MOSFET is operated in an off-state (i.e., the gate voltage does not exceed Vth), and when a sufficient nonzero gate bias voltage is applied with respect to the source and drain, an “accumulated charge” may occur under the gate. The “accumulated charge”, as defined in more detail below and used throughout the present application, is similar to the “accumulation charge” described in the prior art literature in reference to MOS capacitors. However, the prior art references describe “accumulation charge” as referring only to bias-induced charge existing under a MOS capacitor oxide, wherein the accumulation charge is of the same polarity as the majority carriers of the semiconductor material under the capacitor oxide. In contrast, and as described below in more detail, “accumulated charge” is used herein to refer to gate-bias induced carriers that may accumulate in the body of an off-state MOSFET, even if the majority carriers in the body do not have the same polarity as the accumulated charge. This situation may occur, for example, in an off-state depletion mode NMOSFET, wherein the accumulated charge may comprise holes (i.e., having P polarity) even though the body doping is N− rather than P−.
For example, as shown in FIG. 1, when the SOI NMOSFET 100 is biased to operate in an off-state, and when a sufficient nonzero voltage is applied to the gate 108, an accumulated charge 120 may accumulate in the body 114 underneath and proximate the gate oxide 110. The operating state of the SOI NMOSFET 100 shown in FIG. 1 is referred to herein as an “accumulated charge regime” of the MOSFET. The accumulated charge regime is defined in more detail below. The causes and effects of the accumulated charge in SOI MOSFETs are now described in more detail.
As is well known, electron-hole pair carriers may be generated in MOSFET bodies as a result of several mechanisms (e.g., thermal, optical, and band-to-band tunneling electron-hole pair generation processes). When electron-hole pair carriers are generated within an NMOSFET body, for example, and when the NMOSFET is biased in an off-state condition, electrons may be separated from their hole counterparts and pulled into both the source and drain. Over a period of time, assuming the NMOSFET continues to be biased in the off-state, the holes (resulting from the separated electron-hole pairs) may accumulate under the gate oxide (i.e., forming an “accumulated charge”) underneath and proximate the gate oxide. A similar process (with the behavior of electrons and holes reversed) occurs in similarly biased PMOSFET devices. This phenomenon is now described with reference to the SOI NMOSFET 100 of FIG. 1.
When the SOI NMOSFET 100 is operated with gate, source and drain bias voltages that deplete the channel carriers in the body 114 (i.e., the NMOSFET 100 is in the off-state), holes may accumulate underneath and proximate the gate oxide 110. For example, if the source bias voltage Vs and the drain bias voltage Vd are both zero (e.g., connected to a ground contact, not shown), and the gate bias voltage Vg comprises a sufficiently negative voltage with respect to ground and with respect to Vth, holes present in the body 114 become attracted to the channel region proximate the gate oxide 110. Over a period of time, unless removed or otherwise controlled, the holes accumulate underneath the gate oxide 110 and result in the accumulated charge 120 shown in FIG. 1. The accumulated charge 120 is therefore shown as positive “+” hole carriers in FIG. 1. In the example given, Vg is negative with respect to Vs and Vd, so electric field regions 122 and 124 may also be present.
Accumulated Charge Regime Defined
The accumulated charge is opposite in polarity to the polarity of carriers in the channel. Because, as described above, the polarity of carriers in the channel is identical to the polarity of carriers in the source and drain, the polarity of the accumulated charge 120 is also opposite to the polarity of carriers in the source and drain. For example, under the operating conditions described above, holes (having “P” polarity) accumulate in off-state NMOSFETs, and electrons (having “N” polarity) accumulate in off-state PMOSFETs. Therefore, a MOSFET device is defined herein as operating within the “accumulated charge regime” when the MOSFET is biased to operate in an off-state, and when carriers having opposite polarity to the channel carriers are present in the channel region. Stated in other terms, a MOSFET is defined as operating within the accumulated charge regime when the MOSFET is biased to operate in an off-state, and when carriers are present in the channel region having a polarity that is opposite the polarity of the source and drain carriers.
For example, and referring again to FIG. 1, the accumulated charge 120 comprises hole carriers having P or “+” polarity. In contrast, the carriers in the source, drain, and channel (i.e., when the FET is in the on-state) comprise electron carriers having N or “−” polarity. The SOI NMOSFET 100 is therefore shown in FIG. 1 as operating in the accumulated charge regime. It is biased to operate in an off-state, and an accumulated charge 120 is present in the channel region. The accumulated charge 120 is opposite in polarity (P) to the polarity of the channel, source and drain carriers (N).
In another example, wherein the SOI NMOSFET 100 comprises a depletion mode device, Vth is negative by definition. According to this example, the body 114 comprises an N− region (as contrasted with the P− region shown in FIG. 1). The source and drain comprise N+ regions similar to those shown in the enhancement mode MOSFET 100 of FIG. 1. For Vs and Vd both at zero volts, when a gate bias Vg is applied that is sufficiently negative relative to Vth (for example, a Vg that is more negative than approximately −1 V relative to Vth), the depletion mode NMOSFET is biased into an off-state. If biased in the off-state for a sufficiently long period of time, holes may accumulate under the gate oxide and thereby comprise the accumulated charge 120 shown in FIG. 1.
In other examples, Vs and Vd may comprise nonzero bias voltages. In some embodiments, Vg must be sufficiently negative to both Vs and Vd (in order for Vg to be sufficiently negative to Vth, for example) in order to bias the NMOSFET in the off-state. Those skilled in the MOSFET device design arts shall recognize that a wide variety of bias voltages may be used to practice the present teachings. As described below in more detail, the present disclosed method and apparatus contemplates use in any SOI MOSFET device biased to operate in the accumulated charge regime.
SOI and SOS MOSFETs are often used in applications in which operation within the accumulated charge regime adversely affects MOSFET performance. As described below in more detail, unless the accumulated charge is removed or otherwise controlled, it detrimentally affects performance of SOI MOSFETs under certain operating conditions. One exemplary application, described below in more detail with reference to the circuits shown in FIGS. 2B and 5A, is the use of SOI MOSFETs in the implementation of radio frequency (RF) switching circuits. As described below with reference to FIGS. 2B and 5A in more detail, the inventors have discovered that unless the accumulated charge is removed or otherwise controlled, under some operating conditions, the accumulated charge adversely affects the linearity of the SOI MOSFET and thereby increases harmonic distortion and intermodulation distortion (IMD) caused by the MOSFET when used in the implementation of certain circuits. In addition, as described below in more detail, the inventors have discovered that removal or control of the accumulated charge improves the drain-to-source breakdown voltage (i.e., the “BVDSS”) characteristics of the SOI MOSFETs.
Therefore, it is desirable to provide techniques for adapting and improving SOI (and SOS) MOSFETs, and circuits implemented with the improved SOI MOSFETs, in order to remove or otherwise control the accumulated charge, and thereby significantly improve SOI MOSFET performance. It is desirable to provide methods and apparatus for use in improving the linearity characteristics in SOI MOSFETs. The improved MOSFETs should have improved linearity, harmonic distortion, intermodulation distortion, and BVDSS characteristics as compared with prior art MOSFETs, and thereby improve the performance of circuits implemented with the improved MOSFETs. The present teachings provide such novel methods and apparatus.
Gate Oxide Reliability and the Accumulated Charge Regime
The gate oxide is a critical component of a MOSFET. In many applications, including RF switch implementation, it is desirable to make the gate oxide as thin as possible. In RF circuit applications, thinner gate oxide results in higher on-currents and lower insertion losses for RF signals. However, if the gate oxide is too thin, the oxide will break down when a gate voltage is applied. When an electric field is applied to a gate oxide, there is typically a significant time interval before the gate oxide fails. The time required for a gate oxide to fail is a function of the applied electric field and temperature. This phenomenon is known as Time Dependent Dielectric Breakdown (TDDB). As a rough rule of thumb, at room temperature the electric field in a gate oxide should not exceed approximately 5 MV/cm for a desired lifetime or time-to-breakdown of ten years.
TDDB in gate oxides has been investigated extensively. One exemplary reference is an article entitled “A Unified Gate Oxide Reliability Model,” C. Hu and Q. Lu, 37th International Reliability Physics Symposium, San Diego, Calif. 1999. This paper discusses two major mechanisms for TDDB which occur under different stress conditions related to the strength of the applied electric field.
Another exemplary reference is a technical paper entitled “Low Electric Field Breakdown of Thin SiO2 Films Under Static and Dynamic Stress,” J. S. Suehle and P. Chaparala, IEEE Transactions on Electron Devices, Vol. 44, No. 5, May 1997. This reference reports an increase in gate oxide lifetime under bipolar pulsed stress (positive and negative voltage pulses) relative to unipolar (DC) stress. This effect occurs only at very large fields, and is attributed to relaxation of hole trapping occurring in the gate oxide. This phenomenon is unrelated to the improvements in gate oxide reliability that can be obtained by controlling accumulated charge in SOI MOSFETs, as described in more detail herein.
Still yet another reference relating to TDDB is a technical paper entitled “Reliability Issues for Silicon-on-insulator,” R. Bolam, et al., Electron Devices Meeting 2000, IEDM Technical Digest, December 2000. The authors report that there is no significant difference for TDDB failure in bulk Si devices and SOI devices fabricated in accordance with current art. The paper by C. Hu and Q. Lu, the reference by Suehle and Chaparala, and the reference by R. Bolam, cited above (referred to herein as the “TDDB references”), are hereby fully incorporated by reference herein, as though set forth in full for their teachings on the reliability of SiO2 when used as a gate dielectric.
The TDDB references cited above indicate that TDDB lifetime, at a given temperature, is dependent on the electric field in the gate oxide. When charge carriers are present beneath the gate oxide (e.g., when the MOSFET is in an on-state), the electric field in the oxide is approximately equal to the gate-to-source voltage divided by the gate oxide thickness. However, in accordance with teachings presented herein, persons skilled in the arts of electronic devices will appreciate that this is not necessarily the case for an SOI MOSFET operated in an off-state in the accumulated charge regime. In this case, the electric field that stresses the gate oxide is also affected by the presence of an accumulated charge under the gate. In particular, the inventors have discovered that removing or otherwise controlling the accumulated charge can significantly reduce the electric field that stresses the gate oxide and thereby improve the gate oxide reliability. Therefore, it is desirable to provide techniques for adapting and operating SOI MOSFET devices and circuits in order to control the accumulated charge and thereby significantly improve gate oxide reliability. The present teachings provide such novel techniques for adapting and operating SOI MOSFET devices.